HIGH TEMPERATURE PROCESSED NANOSTRUCTURES FOR EMERGING ELECTROCHEMICAL DEVICES
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The ancients exploited fire to keep warm, but temperature embodies more than just warm and cold. Researchers employ high temperatures as a useful tool to develop novel materials. Many reactions require high energy input at the very beginning or during the whole synthesis process. High temperature synthesis is an effective approach to synthesize new materials or to achieve designed phase structures. This approach can be operated under air, a reducing atmosphere, or high vacuum, which are suitable for various applications and are widely used in chemical decomposition and compounding, powder sintering and densifying, as well as nanocrystal growth. In this work, high temperature techniques are applied to two distinct emerging energy-related electrochemical systems, improving the key component material properties for advanced applications. In the first part of the work, a thermal shock approach is employed to synthesize carbon-based composite materials and platinum-group metal (PGM)-free electrocatalysts for oxygen reduction reactions in fuel cells. Specifically, this work has focused on the nanostructure design of graphene/metal nanoparticles to enhance their chemical stability and electrochemical durability during reactions. In addition to the systematic characterization, detailed electrochemical performance evaluations were carried out to get a further understanding of the mechanism, which allows for better materials nanoengineering and amelioration in the resulting devices. The second primary research topic in this dissertation focuses on the high temperature synthesis and post-treatment of high performance Garnet-type solid state electrolytes, which are a promising candidate to replace conventional flammable liquid electrolytes for lithium batteries. A melted lithium-alloying approach is applied to improve the interfacial wettability and stability between the lithium metal anode and Garnet solid-state electrolyte. Thermal shock post-treatment on the Garnet electrolyte is demonstrated to significantly eliminate the impurities and improve the electrochemical properties, such as ionic conductivity. Sol-gel and template methods are carried out to demonstrate a fast synthesis approach to achieve hybrid Garnet electrolytes with excellent flexibility and good performance. Additional investigation includes analysis of the theoretical fracture mechanics of electrolyte as determined by dimensionality. A mechanics-guided strategy is employed to design a composite solid-state electrolyte with superb flexibility.